1. Field of the Invention
The present invention relates to a heat-sensitive recording material making use of the photosensitivity of a diazonium salt. More particularly, the present invention relates to a heat-sensitive recording material containing a diazonium salt with an improved color forming property and storability before use.
2. Description of the Related Art
Recording materials making use of the photosensitivity of diazo compounds are broadly categorized into three types. Materials classified into a first group are subjected to wet development (known as wet-development-type materials), in which a photosensitive layer containing as a primary component, a diazo compound and a coupling component is provided on a support, and the resultant recording material, after being placed on an original, is exposed to light and subsequently developed in an alkaline solution. A second group is known as dry-development-type materials.
Materials classified into this group are different from those of the first group. They are developed with ammonia gas. A third group is known as heat-development-type materials, and includes a type in which an ammonia gas generating agent such as urea, which can generate ammonia gas under the application of heat is contained in a photosensitive layer; a type in which an alkali salt of an acid such as trichloroacetic acid which loses its an acid nature under the application of heat is contained in a photosensitive layer; and a type in which a diazo compound and a coupling component are activated during thermal fusion by use of a higher fatty acid amide serving as a color development aid.
The wet-development-type material has several drawbacks in maintenance such that development solution is required to be replenished or discarded, resulting in being cumbersome in processing the material and in a large apparatus. Further, handwriting cannot be added to a fresh copy immediately after copying because the copy is wet and a copied image cannot endure long-term storage. The dry-development-type material is also problematic in that development solution must be replenished, and a gas absorption apparatus is required in order to prevent leakage of the generated ammonia gas, leading to an increase in size of the apparatus similar to the wet-development-type material. In addition, the dry development-type material has another problem in ammonia odor immediately after copying. In contrast to the wet- or dry-development-type materials, the heat-development-type material does not require development solution and therefore has an advantage in maintenance. However, conventionally known heat-development-type materials have drawbacks such that a high temperature of 150-200.degree. C. is required for development and the temperature has to be controlled within .+-.10.degree. C. so as to avoid insufficient development or change of a color line, resulting in a high cost of the apparatus. Moreover, the high temperature development means that the diazo compounds used must have heat resistance. In many cases, however, heat-resistant diazo compounds do not form satisfactory high-density images. Although many attempts have been made at low temperature development (90-130.degree. C.), reduction in shelf life of the material itself is often seen. Thus, although advantages of heat-development-type recording systems in terms of maintenance when compared to dry-and wet-development-type materials have been well anticipated they have still not attained mainstream in diazo-recording system.
In order to obtain a desired color density by heating a recording material having a support and layers containing diazo compounds and coupling components provided thereon, during the heating process the components must be instantaneously fused, diffused, and reacted to form developed color dyes. During the reaction, the reaction system is preferably made basic so as to accelerate the reaction. Accordingly, in order to produce a photo- and heat-sensitive recording material that can be developed at low temperature and at a recording speed that does not hamper practical use, a basic substance must be incorporated in a coating layer.
Also, there have recently been proposed a photofixing-type heat-sensitive recording material which needs photofixation of an image and which forms an image by utilizing its property of being degraded and deactivated by the action light. Typical materials of this type form images by allowing bdiazo compounds to react with coupling components by the application of heat to thereby form an image, and subsequently, light is irradiated to fix the formed image (Koji Sato, et al., Journal of the Image Electronics Society, vol. 11, No. 4 (1982), pp. 290-296).
In the case of a diazo-type heat-sensitive recording material, coloring of the background portion of the material or during storage before copying or reduction in color density must be suppressed as much as possible.
Thus, a number of attempts have been made to prepare a diazo-type heat-sensitive material that assure a good shelf life and a high recording speed. However, so far no satisfactory materials suitable for practical use have been obtained.
In both heat-development-type diazo copy in material and thermally developable photofixing type heat-sensitive recording materials that develop color upon heating, there is a possibility of causing a color developing reaction during storage at room temperature prior to copying, if the materials are designed so as to be fully developed in color and to form an image having a high density even with low temperature heating. This results in a coloration phenomenon occurring in the background area of the material that must be white. Particularly, when a red-color forming type recording material is used, there is a problem in that a slight coloring (fogging) of the material is quite conspicuous, since the lunsinosity factor is high in this color. In order to solve this problem of apparently compatible with each other, the present inventors conducted extensive studies of recording materials having a support and a heat-developable photosensitive layer, containing a diazo compound, a coupling component, and a basic substance, in which the diazo compound is allowed to contain in a microcapsule. Further, the inventors continued to search for suitable basic substances, and how to make microcapsules, and eventually succeeded in preventing coloring of the background area the material surface during storage of the material prior to use for copying (Japanese Patent Application Laid-Open (JP-A) No. 2-54251). The present inventors also studied color developing aids that accelerate color developing reaction, and found that p-toluenesulfonamide and certain other substances exhibit excellent properties. In transparent overhead projector sheets and recording materials of multicolor recording materials and the like, it is preferable to use components not contained in microcapsules in emulsion form so as to reduce the haze of the recording layer. However, when the aforementioned arylsulfonamide compound is used in the emulsion form, these are problems such that loss of image quality arises due to precipitation of the compound during storage, or color is formed during storage through partial reaction of the compound due to its high water solubility. In order to overcome these problems, the present inventors further continued their research, and found that arylsulfonamides having specific substituents display excellent properties, leading to the present invention.
An object of the present invention is to provide a heat-sensitive recording material having high color developing sensitivity and excellent storability.